Synthesis gases containing hydrogen and carbon monoxide can be purified to produce hydrogen and carbon monoxide products. Additionally, synthesis gas can also be reacted to provide useful chemical composition such as methanol and as a feed to Fischer-Tropsch processes for the production of liquid fuels.
Typically, synthesis gas is produced by steam methane reforming reactions that are conducted by reacting steam and a hydrocarbon containing fuel in the presence of a catalyst to promote the reforming reaction. Since the reaction is endothermic, heat is supplied to sustain the reaction. Steam methane reformers are large complex devices in which a fuel is combusted in the presence of air in order to supply the endothermic heating requirements.
The use of oxygen transport membranes to supply oxygen within reforming reactions has also been proposed in the prior art. The advantage of such a reformer is that the oxy-fuel combustion can be conducted more compactly than air fired combustion and the oxygen is not generated in a separate plant such as one that employs cryogenic rectification. Moreover, the combustion products are principally carbon dioxide and water. The carbon dioxide can be sequestered for its collection or further use as a value-added product.
Oxygen transport membranes employ airtight-ceramic materials, typically perovskites, that exhibit oxygen ion conductivity at an elevated temperature and upon a driving force of an oxygen partial pressure differential on opposite sides of the membrane. In such a device, an oxygen containing feed, for instance air, is contacted on one side of the membrane often referred to as the cathode side. The oxygen ionizes by gaining electrons. The electrons are transported through the membrane material for such purposes. The oxygen ions are transported through the membrane material in a direction opposite to the electrons and emerge at the opposite side of the membrane that is conventionally referred to as the anode side. At the anode side, the oxygen recombines to produce the electrons that are to be transported through the membrane.
An example of the use of an oxygen transport membrane for the production of synthesis gas can be found in U.S. Pat. No. 6,077,323 in which an oxygen transport membrane is employed to separate oxygen from the air to produce oxygen within a reactant section. The permeated oxygen reacts with hydrocarbons within a natural gas and steam containing feed to supply heat to support the steam methane reforming reactions. Such a reactor is consequently known as an autothermal reformer.
The use of oxygen transport membranes within high temperature environments is problematical because the membrane tends to degrade over time and fail. In order to achieve sufficient oxygen flux, the oxygen transport membrane employs a thin dense layer that is supported on one or more porous layers. All layers are typically formed of ceramic materials. One problem that has been found to effect longevity is the fact that over time, the ceramic support will fail by the known action of creep.
Both U.S. Pat. Nos. 5,938,822 and 6,200,541 disclose the use of a metal support layer. The use of a metal support layer is particularly advantageous in that it helps solve yet another problem involving mounting the ion transport element while sealing the element to its mounting, for instance, a tubesheet of a reactor. Metals while not being brittle are also subject to creep at high temperatures. Additionally, in the pure oxygen and highly oxidative environment in which oxygen ion transport elements operate, metals also suffer failure due to oxidation.
In order to overcome durability limitations with respect to oxygen transport materials, it has been proposed in U.S. Pat. No. 6,695,983 to use the oxygen transport membrane at a lower temperature and as an initial stage of the generation of synthesis gas. Thereafter, in a conventional fired reformer located downstream of the oxygen transport membrane reactor, the remainder of the hydrocarbons are reacted with steam to produce the synthesis gas product.
In another attempt to make more durable oxygen transport membrane elements for use in reactors, high temperature alloys have been proposed such as oxide dispersed strengthened metals. As disclosed in U.S. Patent Application Pub. No. 2005/0061633, a composite oxygen ion transport element is disclosed that has a dense layer applied to a support layer having pores of cylindrical configuration. Additionally, a porous layer can be interposed between the dense layer and the support layer to distribute oxygen permeating through the dense layer to the pores of the support layer. The support layer is formed of the oxide dispersed strengthened metal. The problem with the use of such materials within reactors is that they are very expensive.
As will be discussed, the present invention provides a method and apparatus for producing synthesis gas in a reactor that allows the use of expensive high temperature alloys such as oxide dispersed strengthened metal to be utilized in a cost effective manner and that employs robust planar elements that have a wide applicability to any type of device utilizing an oxygen transport membrane.